there's really no way to tell, it depends on your body
if you knew what your natural weight was that might be some indication of where you'll need to get to but if you haven't had a period in 5 years then you probably don't know that
but some people get their periods back at stupidly low BMIs (e.g. 14) while others don't get it til they pass, for example, 22
good luck with your recovery